The higher the difficulty, the longer it'll take for you to mine. The recent 57% jump in difficulty means you'll average out to making 1/1.57 = 0.636 of what you were making before. So if you were making 1.0 BTC yesterday, today you'll be making 0.636 BTC in the same amount of time (disregarding variance).

And in about 10 days, if the difficulty jumps up another 50%, it'll reduce that 0.636 BTC down by a factor of 1/1.5 = 0.666, which would make it 0.4236 BTC. 10 days after that, another 50% jump would reduce it to 0.2821 BTC, then 0.1879 BTC, etc. This is why you should never estimate earnings with no difficulty increases. 50% increases every 10 days is a lot but it hasn't really been letting up. 33% increases every 10 days is probably more reasonable but even at that slower rate, BTC needs to be at least $30 to possibly break even on new rigs.